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Low-Vacuum SEM for Non-Conductive Samples
Non-conductive samples tend to charge under the electron beam, causing image distortion, streaks, and stage drift. This “charging effect” occurs because excess negative charge cannot dissipate through an insulating specimen.
2025-02-09
Taming Charging: Low-Vacuum SEM for Non-Conductive Samples
Non-conductive samples tend to charge under the electron beam, causing image distortion, streaks, and stage drift. This “charging effect” occurs because excess negative charge cannot dissipate through an insulating specimen.
Low-vacuum (variable-pressure) SEM provides a practical way to neutralize surface charge without sputter coating. By admitting a small amount of gas into the chamber, electron–gas collisions create ions that rebalance the surface charge. As a result, imaging becomes stable and details that were previously washed out by charging become visible.
Image: Airlaid paper under different chamber pressures.
At 20 Pa, residual charging is evident—local bright/dark bands and slight blur remain, and edges show halo.
At 60 Pa, charge is well neutralized: fibers are crisp, crossings are clearly separated, and texture contrast is strong.
At 150 Pa, too much gas scatters the beam, reducing signal and SNR, so the image appears softer and grainier.
For this sample, 60 Pa is the optimal setting, balancing charge control and image quality.
Practical tips for low-vac SEM
Start at the lowest pressure that stabilizes the image, then fine-tune in small steps.
Combine moderate kV with higher probe current or longer dwell to compensate for reduced signal.
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